1. Field of the Invention
The present invention relates to the fields of single sideband modulation and shifting the frequency of complex signals of a repetitive or nonrepetitive nature. In particular the invention relates to adding the frequency or phase of one single to another to shift the frequency or phase of said other signal, or frequency addition.
2. Description of the Prior Art
Often in electronic system design, it is desirable to change the frequency of a given complex system, however no such circuitry is believed to exist which can directly effect such a change. As a result the complex signal must be regenerated at a new frequency by various techniques such as tape recording at one speed and playing back at another, or digitizing the signal at a given rate with an A-D converter, buffering in memory and reconstructing the signal via a D-A converter at a different rate. These systems do not however actually change the frequency of the signal, but rather time compress or expand the signal.
Single sideband modulators are known which have the ability of generating a sideband of a carrier in response to a modulating signal. In the SSB modulators, a fixed frequency carrier is modulated in a fashion to generate sidebands which contain both amplitude and frequency information of the modulating signal. Generally, these modulators are utilized to provide voice communications where the modulating signal occupies a voice bandwidth from a few hundred Hz to a few kilohertz. These modulators are useful only when the carrier is considerably higher in frequency than the modulating signal. These modulators are not useful for shifting the phase of the carrier, or for providing slifht frequency shifts to the carrier since the 90 degree phase shifter which operates on the modulating signal does not operate at low frequencies.
A SSB modulator is shown in block form in FIG. 1. It can be seen that a modulating signal 1 is applied to a splitter 3a where it is split and applied to the M input of a first balanced modulator 5a, and via a 90 degree phase shifter 4a to the second balanced modulator 5b. Similarly, a carrier signal 2 is split, and applied to the RF inputs of the balanced modulators 5a and 5b, with the second carier input also being passed through a 90 degree phase shifter 4b. The two signals output from the outputs of the balanced modulators are combined in an adder 6a to create the SSB signal. Because the balanced modulators generate components at odd harmonics of the carrier frequency, a push pull vector type adder is often used for adder 6 in order to minimize the harmonic components. More information on SSB modulators can be found in "Reference Data for Radio Engineers" published by Howard W. Sams & Co., Inc. Indianapolis, Indiana 46268 Copyright 1968. Section 23 contains much information on modulators. An article by Roger Harrison, "A Review of SSB Phasing Techniques," Ham Radio Magazine, (January 1978), provides good information on the subject as well. It might be noted that all of the 90 degree modulating signal (audio) phase shifters shown in this prior art article are AC coupled and/or do not operate at DC. In contrast the present application teaches a method and apparatus to overcome this limitation.
FIG. 2a shows a schematic diagram 7 of a balanced modulator, along with the equivalent circuit 8 in FIG. 2b. A waveform of a typical ouptut is shown in FIG. 2c. Note that the balanced modulator essentially switches the polarity of the modulating signal at the carrier frequency. In other words, the modulating signal is "chopped" by the carrier. No amplitude information of the carrier is passed by the switches, only timing (frequency) information. One wishing more information on balanced modulators, and SSB modulation can refer to the book by Donald G. Fink, "Electronic Engineers' Handbook", (McGraw-Hill, 1975), Section 8-77, is particularly useful.
Clearly, the SSB modulation scheme is not suitable for shifting the frequency of a complex signal by a small amount. It would be possible for the carrier frequency to be shifted by the frequency of the modulating signal having a frequency within the passband of the 90.degree. phase shifter, however no amplitude information on the carrier would be passed by the circuit, due to the switching nature of the balanced modulator. It could be said that the modulating signal is shifted in frequency by the carrier, however the amount of frequency shift is many times the modulating signal frequency, and the system would not work at all for frequency shifts less than the modulating signal frequency. In addition, the SSB modulator generates spurious components at odd harmonics of the carrier frequency, due mainly to the fact that the balanced modulators are switching type devices, and the signal output from the output is essentially a square wave at the carrier frequency which is amplitude modulated. The SSB modulator is not useful for shifting or controlling the phase of a carrier in any fashion partially due to the switching nature of the balanced modulators, and due to the limited bandwidth nature of the 90 degree phase shift network for the modulating signal.